Lead smelting process

ABSTRACT

A process for separately recovering lead values and sulphur values from lead sulphide ores and concentrates in which ore or concentrate in particulate form in admixture with an oxygen-rich gas is charged into a furnace containing a molten bath consisting of lead oxide-containing slag or of lead and a lead oxidecontaining slag cover. Oxidation of the lead sulphide is initiated in a combustion zone above the bath and is completed after oxidized lead sulphide particles have impinged onto and penetrated the surface of the bath. Sufficient oxygen-rich gas is supplied to provide the heat of oxidation to maintain a flame in the combustion zone at a temperature of at least 1,300*C. and to maintain said bath at a temperature of at least 1,100*C. Oxygen supplied to the furnace maintains a lead content of at least about 35 percent, as lead oxide, in the slag in order to maintain slag fluidity and to keep the sulphur content of the slag at a low level, thereby assuring production of low-sulphur lead values. Sulphur values are recovered as concentrated sulphur dioxode.

United States Patent 11 1 Liang et al.

[ Nov. 12, 1974 LEAD SMELTING PROCESS {22] Filed: May 7, 1973 [211 App].No.: 357,536

Related US. Application Data [63] Continuation-impart of Ser. No.50,749, June 29,

I970, abandoned. v

[52] U.S. Cl 75/77, 75/78, 423/89 [51] Int. Cl C22b 13/00 [58] Field ofSearch 75/77, 78, 63; 423/89 [56 1 References Cited UNITED STATESPATENTS 3,300,301 l/l967 Malmstrom 75 77 3,326.67] 6/1967 Worner 1. 7577 X 3,281,237 10/1966 Meissner et al 75/77 Primary Examiner-Herbert T,Carter [57] ABSTRACT A processfor separately recovering lead values andsulphur values from lead sulphide ores and concentrates in which ore orconcentrate in particulate form in admixture with an oxygen-rich gas ischarged into a furnace containing a molten bath consisting of leadoxide-containing slag or of lead and a lead oxidecontaining slag cover.Oxidation of the lead sulphide is initiated in a combustion zone abovethe bath and is completed after oxidized lead sulphide particles haveimpinged onto and penetrated the surface of the bath. Sufficientoxygen-rich gas is supplied to provide the heat of oxidation to maintaina flame in the combustion zone at a temperature of at least l-,300C. andto maintain said bath at a temperature of at least l,lOOC. Oxygensupplied to the furnace maintains a lead content of at least about 35percent, as lead oxide, in the slag in order to maintain slag fluidityand to keep the sulphur content of the slag at a low level,

- thereby assuring production of low-sulphur lead val- 19 Claims, 1Drawing Figure PATENTED NOV 12 1974 This application is acontinuation-in-part of application Ser. No. 50,749 filed June 29, 1970,now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to an improvedprocess for the smelting of lead sulphide ores and concentrates.

The earliest known process for smelting lead sulphide ore involvedoxidizing the ore on an open grate. The principal reactions taking placein such a smelting operation are as follows:

PbS Pb S0 2PbS 30 2PbO 280 The metallic lead so formed in accordancewith Equation I was collected for subsequent refining while the leadoxide slag was separately reduced, for example, using coke, according tothe following equation:

2PbO c 2Pb co III form an intermediate material containing a majorproportion of lead oxide according to Equation II. Such roastingreactions have, for instance, been carried out so as to provide a leadoxide sinter which is subsequently transferred to a blast furnace forreduction with coke according to Equation III.

The well known roaster-reaction process differs from the roast-reductionprocess in that the initial oxidation is only partial so as to providean intermediate material containing lead oxide according to Equation ll,unconverted lead sulphide, and lead sulphate according to the equation:

PbS PbSO,

The resulting intermediate material may then be heated at an appropriatetemperature essentially in the absence of .oxygen to cause reaction tooccur between the unconverted lead sulphide and the oxidized productsaccording to the following equations:

PbS PbSO 2Pb 2SO One might consider as a roast-reaction process one thatinvolves charging the lead sulphide ore to a reverberatory furnace inwhich it is heated with a supply of air to form such an intermediateproduct, the air supply being then discontinued and the furnacetemperature increased to cause the reactions of Equations V and VI totake place, with metallic lead and sulphur dioxide produced as endproducts. It will be appreciated that, unless the lead sulphide on theone hand, and the lead oxide and the lead sulphate on the other hand,are present in exactly stoichiometrically equivalent amounts in theintermediate material, there will be either an excess or a deficiency ofavailable oxygen in the second stage of such a roast-reaction process.This is a difficult balance to maintain; Therefore, roast-reactionprocesses have been developed so as to be operated deliberately witheither an excess or a deficiency of oxygen.

In the process of US. Pat. No. 2,416,628, lead sulphide concentrate ismelted and partially oxidized in an electric furnace. Oxygen is providedby blending the concentrate with oxidic materials such as recycled fumeand partially roasted concentrate containing lead oxide and leadsulphate. By avoiding the use of oxidizing gases, and by the formationof a low vapor pressure Pb-PbS solution, volatilization of lead withoff-gases is substantially decreased in this stage. However, subsequentair blowing of the Pb-PbS solution in a separate furnace, even under amolten glass cover, was found to result in volatilization of as much as40 percent of the lead requiring subsequent recovery in a gas purifyingplant and recycle of the flue dust by mixing it with feed to theelectric furnace.

Operation with close to stoichiometrically equivalent amounts ofsulphidic and oxidic feeds is the object of US. Pat. No. 2,797,158.Alternate batches of sintered material, one with an oxygen deficiencyand the next with an oxygen surplus, are obtained by roasting leadsulphide concentrate admixed with oxidized material in appropriateproportions. Several such batches are combined to form a charge which isheated to a temperature between 1,100C. and -l,250C. to produce metalliclead and volatile sulphur compounds as set forth in the reactions ofEquations V and VI.

In the operation of the process of US. Pat. No. 2,797,158, as furtherillustrated in Transactions of the Metallurgical Society of AIME, 224,1962, pages 939-944, a circulating load of zinc increases continuouslyin the fume that evolves from the heating of the sintered material.Although Canadian Pat. N 0. 574,935 provides for the removal of thiszinc, which otherwise makes fluidity of the slag difficult to maintain,separate blast furnace or hydrometallurgical treatment of the fume isrequired.

All the operations hereinbefore described have the disadvantages-thatmaterial must be transferred physically between the separate stages andthat full advantage cannot be taken of the exothermic nature of thereaction between lead sulphide and oxygen. Consequently,-fuel orelectrical energy is required during the second stage to melt theintermediate reaction product i and to sup'ply the heat required for theendothermic reactions between lead sulphide, and lead oxide andsulphate. Moreover, separate discharging of gases from the two stagesand the dilution of the sulphur dioxide with gaseous combustion residuescomplicates the recovery of said sulphur dioxide. Additionally, highgrade concentrates containing as much as 80 percent lead sulphide whichare presently available from flotation units present certain problemswhen treated on conventional sintering machines. To overcome theseproblcms, it has been necessary to dilute such concentrates withslag-forming materials and/or with recycled material and fly ash, or touse specially designedupdraft sintering machines In order to avoid thesedifficulties, several proposals have recently been made for theconversion of leadsulphide ores and concentrates in single stageoperations. Most of these recent proposals have involved feeding thelead sulphide and an oxygen-containing gas such as air to a furnace toobtain the direct conversion to metallic lead in accordance withEquation 1.

To operate known direct smelting processes efficiently, it appearsessential that the lead sulphide and the oxygen-containing gas beintroduced into the furnace in intimate admixture with each other and insuch relative proportions that as little lead oxide as possible isformed by the reaction of Equation II. Shortcomings such as batchoperation, need for external heat, dilution of sulphur dioxideby-product and refractory failure are still encountered in these leadsmelting processes.

SUMMARY OE THE INVENTION By the process of the present invention,several unexpected advantageous results may be obtained under theconditions of operation, including the use of an oxygenrich gas which,combined with the available oxygen content of solid oxidic material,such as PbO and PbSO that may be charged to the process, is sufficientto ensure an excess of oxygen over that stoichiometricallyrequired forconversion of the lead sulphide completely into metallic lead andconversion of other sulphides, e.g., zinc sulphide, if present, tooxides. With this excess of oxygen, a slag is formed containing anexcess of lead oxide which, according to the reaction of Equation V,drives off, as sulphur dioxide, sulphur contained in lead sulphideentering the molten slag. As a result, the lead settlinginto the bullionis very low in sulphur. We have found, with a slag containing at least35 percent lead as lead oxide and kept above l,lOC., such sulphurrejection is assured. Substantial recoveries of low-sulphur lead bullionwere obtained with slags containing as much as 55 percent lead as leadoxide.

In accordance with the present invention, the lead sulphide ore orconcentrate is charged in particulate form and in admixture with saidoxygen-rich gas, preferably containing at least 75 percent oxygen,downwardly through one or more nozzle feeders into a furnace whichcontains a molten bath consisting of lead and a lead oxide-containingslag cover. Oxidation of the downwardly moving charge of lead sulphideis initiated inside the furnace prior to forceful impingement of theparticulate material onto the molten bath and penetration of the surfacetherof, and oxidation continues after such impingement and in contactwith the molten bath. The forceful impingement of particles on themolten bath serves to ensure completion of the desired reactions whichare believed to proceed according toEquations I, [I and V. The furnacethus is operated to ensure that the formation of PbSO isthermodynamically unfavorable in the gas and slag phases in thecombustion zone of the furnace,'thereby avoiding the reaction ofEquation IV.

The process of the invention is intended to be operated continuously butmay be operated batchwise in conjunction with a reduction step in thesame furnace. The removal of molten lead and slag material from thesmelting furnace may be intermittent if desired. The slag, for example,may be withdrawn from the furnace either intermittently by the provisionof a suitable taphole or continuously by the provision of a slagoverflow weir. The lead oxide-containing slag can be subsequentlyreduced to recover the leadtherefrom.

The lead obtained from the furnace is soft lead, low in arsenic andantimony. Operating experience shows that essentially all of the arsenicand antimony in the feed reports to the slag.

Special refractories are not required for the furnace. A good grade ofchrome-magnesite brick provides a satisfactory lining. The furnace iscompact because of the short distance between the feeder nozzle and themolten bath. The concentrate or ore feeder system provides sufficientmixing with the oxidizing gas to give ignition and a sustained flame.The furnace may be rectangular or oval in shape with one or moreconcentrate or ore feeders.

The furnace is operated with a bath temperature of 1,1001,300C. althoughthe temperature in proximity to the stream of material impinging on themolten bath is normally above 1,500C. High temperature of the combustionflame prevents the formation of PbSO which is unstable at 0.1 atmosphereoxygen partial pressure at temperatures above 1,300C. The point ofimpingement and the position of the flame should be far enough from thefurnace walls to minimize damage to the refractory lining on the walls.The nozzles through which the feed stream enters the furnace should bedimensioned to provide the required supply rate and should be disposed asufficient distance above the surface of the molten bath to permitadequate oxidation of the lead sulphide prior to impingement, and toavoid nozzle plugging by accretions.

Although thermodynamic calculations indicate that it is technicallypossible to use oxidizing gas with as low as 60 percent oxygen in thisprocess, heat .retention in the furnace is considerably imporved by theuse of gas containing at least percent oxygen, so as to reduce the heatand dust losses from the furnace due to the throughput of inertgases,e.g., nitrogen. Such use of oxygen produces exhaust gases having ahigh concentration of sulphur dioxide. At this concentration, sulphurdioxide can be recovered more readily than at concentrations formed byoxidation with air.

In order to obtain a substantial lead fall in the furnace as low sulphurbullion, operation with a slag containing from about 35 to 55 percentlead in the form of lead oxide is preferred. Slag containing less leadpermits too much sulphur to enter the bullion. Because of a higherproportion of zinc and iron,.Which is present in the ferric form,additional flux is required to maintain fluidity of slags that containless than 35 percent lead. It is not necessary to have more than about55 percent lead in the slag for production of a low sulphur bullion.However, it may be convenient, as hereinafter explained, to recover aslead oxide in a low sulphur slag, all, or nearly all, the lead that isnot given off as fume.

The particle size normally found in commercial lead concentrates, forexample flotation products, is suitable for the combustible feed to thefurnace.

BRIEF DESCRIPTION OF THE DRAWING DESCRIPTION OF THE PREFERREDEMBODIMENTS With reference to the drawing, it will be seen that arectangular furnace indicated generally therein at 10 has a slopingfloor 12 extending from a lead well14 at one end of the furnace to arestricted slag overflow weir 16 at the other end. A rising passage 18extends upwardly and outwardly from the lead well 14 through the endwall of the furnace 10 to provide a seal and to permit the overflow ofmolten lead from the furnace into a lead-receiving vessel 20. An exhaustflue 22 extends through the roof of the furnace to a dust recovery unitand a sulphur dioxide recovery system, not shown. A slag-receivingvessel 17 is shown for receiving the slag material from the slagoverflow weir 16.

At one end of the furnace, two feed nozzles 24 and 26 extend through theroof of the furnace 10. The nozzle 24 is supplied by a line 30 with anoxygen-rich gas into which particulate lead sulphide ore or concentrateis introduced by a screw conveyor 32 from a concentrate hopper 34.-Thenozzle 26 is fed from hopper 38 by a screw conveyor 36 with particulateoxidic material which may include recycled fume, flux-forming materialand sulphate leach residues from an electrolytic zinc plant. As thetemperature of the slag and its lead content are higher than in theconventional processes, the slag, to be sufficiently fluid, requiresonly a small input of siliceous flux material, if any, thus providingthe advantage of a lower slag volume. Flux material is fed, if required,from the feed hopper 38 by screw conveyor 36 into nozzle 26 to maintainthe required slag fluidity.

In operation, the furnace 10 contains a molten bath 48 consisting ofmolten lead 40 covered by a molten slag layer 42 which contains leadoxide. The particulate lead sulphide ore or concentrate in intimateadmixture with gaseous oxygen is introduced into the furnace through thenozzle 24 and the stream 43 issuing from this nozzle ignites in thespace above thebath 48 to provide a high temperature flame in combustionzone 44 in which part of the oxidation of the lead sulphide occurs. Inpractice, temperatures as high as l,700C. or higher are obtained withinthe flame. Such temperatures were estimated -by thermodynamiccalculations. Temperature readings could notbe obtained withthermocouples sheathed in stainless steel, which has a melting point ofabout 1,500C. Rapid destruction of these thermocouples indicated flametemperatures substantially above l,5( )0C. Presence of fume precludedthe use of optical devices;

Before the oxidation of the lead sulphide particles is complete, theflame forcefully impinges-on the molten bath 48 disturbing the slaglayer42 as shown at 46. This forceful impingement on the molten bath servesto allow reactions according to Equations I, II and V to reachequilibrium at an enhanced'rate. The injection feed nozzle 24 isdisposed inwardly from the furnace walls to reduce damage by the hightemperature flame to the refractory linings of these walls. The distancefrom the discharge end of the nozzle 24 to the surface of the moltenbath is sufficient to prevent plugging of the nozzle by accretions andto permit sufficient oxidation of the lead sulphide concentrate prior toimpingement in order to get a bath surface temperature of at leastl,l00C. but short enough to ensure that the desired forceful impingementis obtained. The distance from the nozzle to the bath surface isdependent on the size of furnace, tonnage of concentrate treated andvelocity of the downward stream. For a pilot plant of the type shown inthe attached drawing, and treating ten tons of concentrates per day, 36inches were effective. For larger furnaces, this distance should beincreased to a range between 4 feet and 7 feet.

Lead formed during the oxidation enters the molten lead and the leadwell 14 to be removed from the furnace 10 through the overflow passage18 away from the area of impingement. Some of the lead oxide formedduring the oxidation enters into the slag layer 42 and overflows at 16also away from the area of impingement, into the slag-receiving vessel17. The slag may be treated in a reduction furnace to recover its leadcontent. The sulphur dioxide formed, any unreacted oxygen, other gaseousproducts and fume rich in lead oxide leave the furnace via the exhaustflue 22 for subsequent treatment.

The temperature of the molten slag bath 42 during operation is in therange of from about 1,100C. to about 1,300C. except in the area ofimpingement 46 where it is impinged upon by particles from the hottercombustion zone, in which area the temperature is somewhat higher. Thelead oxide-containing slag, which has an exceptionally low sulphurcontent and which leaves the furnace at opening 16 beyond the area ofimpingement, passes to a reduction furnace in which this slag can bereduced in a conventional manner such as smelting with the addition of asuitable reducing agent such as coke.Most of the fume entrained in the Yoff-gases is recovered easily and returned to the furnace. Fume settleswell by gravity if the flue 22 is sufficiently large in cross section toprovide low gas veloc ity, thereby reducing the load on separatingapparatus such as a bag-house or an electrostatic treater. Fume that iscollected outside the furnace as flue dust may be returned to thefurnace as it is collected. Agglomeration of dust particles and blendingwith lead sulphide feed are not required. Higher temperatures areobtained in the pre-impingement combustion zone 44 if the flue dust andother oxidic material, which reactendothermically, are not fed into thefurnace through the nozzle 24. It is preferred that nozzle 26 be used,whereby the flue dust and'other oxidic materials are fed into thefurnace ,in the stream designated by numeral 47 in proximity tooradjacent to stream 43. The portion of the fume that falls freely inthe flue drops onto the bath surface below the flue', where leadsulphate in the fume decomposes by reaction, 'according to Equation V1,with lead sulphide also contained in the fume. Also, lead sulphate inthe fume decomposes thermally when the fume falls on the part of thebath, near the combustion zone, that is above 1,300C. The gas streamfrom the furnace 10 passes from the dust separator to a conventionalrecovery system where its sulphur dioxide content is recovered as aproduct, for example, liquid 80,.

For controlled operation of the furnace, the oxygenrich gas with theparticulate feed is admitted at a prede termined rate. This oxygen,which must be sufficient to maintain the furnace temperatures, is alsothe principal supply of oxygen that is available to form the lead oxidein the slag. When substantial recoveries of metallic lead bullion are tobe obtained, the required amount of oxygen-rich gas may be estimated asa percentage of the calculated stoichiometric requirement for treatmentof lead concentrate to convert the lead sulphide completely to lead andsulphur dioxide. This oxygen requirement depends on the composition ofthe lead concentrate feed, allowance being 'made'for oxygen used toconvert Zinc sulphide and iron sulphide to oxides and forlead-containing, oxidic material that may be separately charged to theprocess. When substantial recoveries of lead bullion are obtained,inward leakage of air into the furnace, which provides hygienicallydesirable operation under slightly less than atmospheric pressure, issmall and may be neglected for this calculation, which is illustrated byexample in Table 1.

TABLE I Example Calculation of Oxygen Requirement for 100 Pounds of LeadConcentrate Containing 74% Pb, 4% Zn, 3% Fe.

Reaction stoichiometric O Gaseous oxygen used is x% of 17.4 pounds, anempirically determined value of x being chosen to provide sufficientlead oxide in the slag to ensure a low level of sulphur in the bullionand to keep the slag temperature above 1,100C. This value depends onoperating conditions, mainly composition of solid feed and handling ofevolved fume. if little or none of the fume, which contains availableoxygen, mainly as lead oxide, is returned to the process, there will beinsufficient lead oxide in the slag to ensure production of low sulphurbullion unless a relatively large x factor is used. Operation with alarge X factor increases lead in the slag at the expense of lead in thebullion. Recycling of much or all of the culated stoichiometricrequirement for treatment of the lead concentrate to convert the leadsulphide completely to metallic lead and sulphur is sufficient tomaintain bath and combustion zone temperatures and to provide a slagthat contains at least 35% lead as lead oxide and is substantiallysulphide free. A preferred range between 102 'and percent of thecalculated stoichiometric requirement ensures substantial recovery oflead as low-sulphur'bullion. Oxygen additions near percent, althoughdirecting more lead into the slag as lead oxide, are of considerablevalue for short time periods during which increased heat output isneeded to maintain control of furnace temperature. Rapid consumption offree oxygen in the preimpingement reaction assures an oxygen partialpressure that is too low for the formation of stable PbSO A lowsulphurslag, from which lead is recovered by subsequent reduction, is formed.All the heat for the process is provided by the oxidation reaction,except during start-up operations when the furnace is heated initiallyby using natural gas or oil.

The following examples present results obtained in the operation of theprocess of the present invention in a 10 tons-per-day pilot plant. Thisoperation produced exhaust gas containing more than 85 percent sulphurdioxide. it will be understood, however, that the scope of the inventionis not restricted to these examples.

EXAMPLE 1 A lead concentrate was admixed with a gas containing 97percent oxygen and charged into a furnace similar to that shown in theFIGURE. The concentrate contained 63.3% Pb and 18.3% S with sulphides ofiron and zinc as the main impurities. The concentrate feed rate was 9pounds per minute. The oxygen supply with the concentrate feed was 106percent of the calculated requirement to convert all the lead sulphideto metallic lead. No flux materials were used and no flue dust wasrecycled to the furnace. The distribution of lead, by weight, in theconcentrate, slag, metal and flue dust was as follows:

Concentrate Slag Lead Bullion Flue Dust Lead (1b.) 36,000 17,000 7,400 Il 1,200

' Product analyses, weight per cent, are tabulated.

Pb Fe Zn S As Sb CaO SiO Slag 51.5 19.9 8.6 0.2 0.18 0.23 0.84 2.9 Lead0.07 0.0006 0.007 Flue Dust 77.4 3.7 3.9 5.0 0 25 0.12 0.3 0.4

fume reduces or eliminates oxygen lo'ss via fume, and effectiveoperation is obtained with a lower x factor. Addition of solid oxidiclead containing material, not

' derived from the measured output ofconcentrate, per- The slagcontained 0.1 percent sulphate sulphur and had an average temperature of1,280C. The exhaust gases contained 84% S0 1.1% 0 9% N 3% H 0 and 1% C0The flue dust contained 2.5% sulphate sulphur.

EXAMPLE 2 A lead concentrate was charged, with gas containing 97 percentoxygen, into the furnace. The concentrate contained 72.5% Pb and 17.2% Swith sulphides of iron and zinc as the main impurities. The concentratefeed rate was 1 1 pounds per minute. The oxygen supply with ConcentrateSlag Lead Bullion Fluc Dust Lcad (11).) 34,000 6,300 19,200 8,000

Product analyses, weight per cent, are tabulated.

flue dust contained 3.4 percent sulphate sulphur.

EXAMPLE 4 A lead concentrate was charged, with gas containing 97 percentoxygen, into the furnace. The concentrate contained 74.5% Pb and 16.2% Swith sulphides of iron and zinc as the main impurities. The concentratefeed rate was 14.5 pounds per minute. The oxygen supply with the feedwas 102.3 percent of the calculated requirement to convert all of thelead sulphide to metallic lead. Siliceous flux containing 81% SiO wasfed into the furnace at 18 pounds per hour. ln this case, 95% of theflue dust was recycled to the furnace. The distribution of lead, byweight, in the concentrate, slag metal and flue dust was as follows:

Pb Fe Zn S As $1) CaO S102 Slag 38.7 13.1 15.8 0.4 0.020 0.029 4.9 12.0Lead 028 0.0o01 0.0015 Flue Dust 77.7 1.1 1.9 7.6 0.006 0.004 0.35 0.2

The slag contained 0.05 percent sulphate sulphur and Concentrate SlagLead Bullion Flue Dust 4 O had an average temperature of 1,150 C. Theexhaust Lead ("1) 98,500 24000 73000 300 gases contained 88% S0 0.06% 02.3% N 3% H 0 and 3% C0 The flue dust contained 3.4 percent sulphatesulphur.

Product analyses, weight per cent, are tabulated.

EXAMPLE 3 A lead concentrate was charged, with gas containing 97 percentoxygen, into the furnace. The concentrate contained 74.1% Pb and 16.1% Swith sulphides of iron and zinc as the main impurities. The concentratefeed rate was 10.2 pounds per minute. The oxygen supply with the feedwas 100.2 percent of the calculated requirement to convert all of thelead sulphide to metallic lead. Siliceous flux containing 81% SiO wasfed into the furnace at pounds per hour. In this case, 85 percent of theflue dust was recycled to the furnace. The distribution of lead, byweight, in the concentrate, slag, metal and flue dust wasas follows:

Concentrate Slag Lead Bullion Flue Dust Lead (10.) 54,000 13,000 36,4002,100

Product analyses, weight per, cent, are tabulated.

The slag contained 0.1 percent sulphate sulphur and had an averagetemperature of 1,180C. The exhaust gases contained 87% S0 0.5% 0 6% N 3%H 0 and 3% C0 The flue dust contained 3.3 percent sulphate sulphur.

The foregoing analyses indicate that operation at a combustion zonetemperature above that at which lead sulphate decomposes and themaintenance of a relatively high level of lead oxide in the slag provideeffective decomposition of lead sulphide by the reactions of Equations Iand V. The differences between the total sulphur and the sulphatesulphur analyses of the flue dusts in the foregoing examples indicatethat about half the total sulphur is in the sulphide form. It appearsthat, when fume falls onto the bath surface, which is above 1,100C.,elemental lead and sulphur dioxide are formed by the reaction accordingto Equation Vl. These factors ensure production of low-sulphur slag anda low-sulphur bullion. The relatively high level of Pb Fe Zn S As Sb Ihad an average temperature of 1,160C. The exhaust gases contained 88% S03% H 0 and 3% C0 The The amount of lead recovered in the flue dust,mainly as lead oxide, i.e., 20 to 30 percent of the lead in thepreliminary sintering step. Return to the furnace of lead oxide fume,recoverd with relative ease from the exhaust gases, contributessignificantly to the output of lead product without substantiallyincreasing the lead oxide content of the slag. As indicated in Example4, the lead content of the flue dust recycled to the furnace, estimatedas 20,000 pounds of lead, is of the same order of magnitude as the24,000 pounds of lead reporting to the slag. With recycling, anequilibrium is established. The reaction according to Equation V ispromoted. Lead oxide from the fume reacts with lead sulphide, also inthe fume, to yield lead and sulphur dioxide. Impurities such as zinc,arsenic and antimony, evolved in the initial reaction, do not build upin the fume.

It should be noted that, with a substantial amount of lead reporting asoxide to the slag in all the examples, the method of calculating thestoichiometric oxygen requirement allows for the presence of an excessof oxygen in the process as it is operated. With admission of smallexcesses of inlet oxygen through the feed nozzle,

' as calculated for Example 3, or even with small apparent deficiencies,occurrence of a high content of lead oxide in the slag may beunexpected. However, there was sufficient oxygen available to producethe high lead oxide slag because small amounts of unoxidized sulphidewere retained in the slag and bullion, some iron occurred in the slag asFe O rather than Fe O and a small but significant input of oxygenoccurred through operation of the furnace at less than atmosphericpressure to fulfill hygienic requirements. Nitrogen analyses of the exitgases show some net input of oxygen by meansof air leakage into thefurnace.

Example 4, with 102 percent of the calculated stoichiometric requirementof oxygen in the inlet gas, produced lead bullion with 0.25 percentcompared with 0.56% S for Example 3, which had 100 percent of thecalculated oxygen requirement in the inlet gas. In order to ensure a lowsulphur content in the bullion, operation with at least 102 percent ofthe calculated oxygen requirement in the inlet gas is preferred.However, projections of test data show that, by permitting increasedsulphur in the bullion, operation of a furnace is feasible with agaseous oxygen supply that is as low as 98 percent of the calculatedrequirement to convertthe sulphide to lead metal. With incidental oxygenadditions as noted above, in quantities that are not readilypredictable, sufficient lead oxide will form in the slag to maintainfluidity and to promote decomposition of all but a small quantity oflead sulphide by the reaction of Equation V. A slag temperature that isabove l,lC. will ensure low sulphate sulphur.

When operating a furnace large enough to treat 100 tons per day of leadconcentrate we observed that a substantial portion of the oxygenrequired to form a Y slag containing at least 35 percent lead as leadoxide may be supplied by means other than the oxygen introduced with theparticulate feed. This is illustrated in Example 5 in which no leadbullion was formed. All the lead values in the lead sulphide in the feedwere converted to lead oxide-containing fume and low sulphur, leadoxide-containing slag. In this example, the quantity of oxygen-rich gasin admixturewith the particulate feed was sufficient to maintain thetemperatures required in the process, but was less than the quantitystated in each of the preceding examples.

EXAMPLE 5 A lead concentrate was admixed with a gas containing 97percent oxygen and charged into a furnace similar to that shown in theFIGURE. The concentrate contained 65.5% Pb, 18.0% S, 9.8% Fe and 3.5% Znwith sulphides of iron and zinc as the main impurities. The concentrate,122,828 pounds, was fed at a rate of 98 pounds per minute. The oxygensupply in admixture with the particulate concentrate was 83 percent ofthe calculated stoichiometric requirement to convert all the leadsulphide to metallic lead. No flux materials were used and no flue dustwas recycled to the furnace. In this test, lead bullion did not form andonly 25 percent of the exhaust gas was sulphur dioxide. It was evidentthat a substantial amount of oxygen entered the furnace as air by meansother than nozzle 24.

Concentrate Slag Lead Bullion Flue Dust Lead (1b.) 80,450 57,550 22,750

Product analyses, weight per cent, are tabulated.

had an average temperature of 1,150C. The exhaust gas, although lower inS0 than the exhaust gases of Examples 1 to 4, stillexceeded theconcentration, about 12 percent, that may be used for the manufacture ofsulphuric acid. N I

This example shows that the gaseous oxygen that entered the furnace inadmixture with the particulate feed was sufficient to maintain the slagtemperature above l,l00C. However, this amount of oxygen, 83 percent ofthe calculated stoichiometric requirement to convert all the leadsulphide to metallic lead, was not enough to convert all the lead tolead oxide. With this 100 tonsper-day furnace, inward leakage of air wassubstantially greater than with'the tighter l0 tons-per-day pilotfurnace which, in Examples 1 to 4, produced exhaust gas containing morethan S0 We have, therefore,

found that a portion of the oxygen requirement to maintain the desiredlead oxide level in the slag can be provided from an extraneous sourcesuch as by inward leakage of air, while the oxygen entering the furnacein admixture with particulate feed maintains temperatures that aresufficiently high to ensure low-sulphate sulphur in the slag.

Oxygen supply must be adequate to obtain the lowsulphur slags disclosedin Examples 1 to 5 and the lowsulphur lead bullions disclosedin Examples1 to 4, and to provide the elevated operating temperatures that ensurelow sulphate sulphur in the slags. To meet these specific oxygenrequirements, there must be, firstly, sufficient oxygen in the form ofoxygen-rich gas containing at least 60% oxygen in admixture with theparticulate feed to initiate and sustain oxidation of the lead sulphidein a combustion zone flame having a temperature above l,300C. and toprovide sufficient heat of oxidation to maintain the molten bath at atemperature above l,lC. and, secondly, sufficient total oxygen tomaintain a slag that is sufficiently high in lead as lead oxide to reactwith impinging lead sulphide and to reject, as sulphur dioxide, thesulphur from lead sulphide that enters the slag. Example 2 indicatesthat a level of about 35 percent lead in the slag is sufficient to meetthe lead oxide requirement. Examples 1, 3 and 4 show that, with lead inthe slag as high as 55 percent, there are still substantial recoveriesof lead as bullion. Example 5, with 0.3 percent total sulphur including0.l percent sulphate sulphur in the slag, shows that the process may beoperated to recover in a low-sulphur slag, all, or nearly all, of thelead values that are not evolved withthe fume.

Operation of this process with at least 35 percent lead as oxide in theslag will produce slag and bullion that are low in sulphur. In order towithdraw from the furnace a substantial portion of the lead as metalliclead bullion, operation with a slag containing from about 35 percent toabout 55 percent lead is preferred. For operation of a furnace that hasa low inward leakage of air, as indicated by at least 80% S0 in theexhaust gas in Examples 1 to 4, it is desirable to control recovery oflead as bullion by introducing a predetermined quantity of oxygen-richgas with the particulate feed. For operation of a furnace that has agreater inward leakage of air, sufficient oxygen must be supplied toform a slag that contains sufficient lead oxide to react with and rejectsulphur, as sulphur dioxide, from the sulphides contained in theimpinging particles. The temperature of the slag must be high enough todecompose sulphates entering the bath as settled fume containing bothsulphide and sulphate sulphur, or as recycled fume and other oxidicmaterials charged to the furnace near the combustion zone. Maintenanceof at least 35 percent lead as oxide in the slag and maintenance of theslag temperature above l,l00C. will prevent entry of sulphur into anunderlying layer of lead bullion. Operation to oxidize all theconcentrate lead values to fume and low-sulphur slag, as in Example 5,and subsequent reduction of the slag to recover metallic lead, offerecological advantages over sintering of concentrate and reduction of thesinter.

Although the foregoing smelting process is preferably operated withsufficient oxygen to form a slag containing the 35 to 55 percent leadrequirement which ensures substantial recovery of bullion low insulphur, arsenic and antimony, or with sufficient oxygen to convert thelead sulphide completely to oxide and sulphur dioxide, it is evidentthat, as the degreeof oxidation increases, lead values within the bathwill have decreasing proportions of lead as bullion.

What we claim as new and desire to protect by Letters Patent ofthe'United States is:

1. A process for separately recovering lead values and sulphur valuesfrom a lead sulphide ore or concentrate without prior sintering whichcomprises: charging lead sulphide ore or concentrate in particulate formand in admixture with an oxygen-rich gas containing at least about 60percent oxygen downwardly into and through the combustion zone .of afurnace to impinge cess, whereby oxidation of said lead sulphide isinitiated and sustained to produce lead oxide and sulphur dioxide in aflame in said combustion zone above the area of impingement onto saidmolten bath and, after such impingement, oxidation of residual leadsulphide is continued by reaction within the molten slag; supplying inthe furnace sufficient oxygen gas in admixture with the ore orconcentrate to provide heat of oxidation to maintain said flame at atemperature of at least l,300C. and said molten bath at a temperatureabove l,l00C., and sufficient total oxygen to maintain at least 35percent lead as lead oxide in the slag, said heat of oxidation beingprovided by reaction with said oxygen-rich gas; and withdrawing fromsaid furnace molten material containing lead values low in sulphurcontent as molten lead oxide-containing slag plus any molten lead thatmay be formed and gaseous material containing sulphur values andentrained lead oxide fume.

2. A process as claimed in claim 1 in which said process is conductedcontinuously.

3. A process as claimed in claim 1 in which from about 35 percent toabout 55 percent lead as lead oxide is maintained in the slag.

4. A process as claimed in claim 1 in which lead values contained in themolten material withdrawn from the furnace include soft lead containingless than 0.001 percent arsenic and less than 0.01 percent antimony.

5. A process as claimed in claim 2 said molten bath consisting of leadand a substantially sulphide free lead oxide-containing slag cover, saidoxygen-rich gas providing at least about 98 percent of the calculatedstoichiometric requirement of oxygen for treating said lead sulphide ore.or concentrate to convert the lead sulphide completely tometallic leadand sulphur dioxide, and separately withdrawing from said furnace moltenlead oxide-containing slag, molten lead, and a mixture of gaseousmaterial and entrained fume.

6. A process as claimed in claim 2 in which said oxygen-rich gasissupplied in an amount sufficient to provide oxygen corresponding tofrom about 98 percent to about 120 percent of the calculatedstoichiometric re quirement.

7. A process as claimed in claim 2 in which said oxygen-rich gas issupplied in amount sufficient to provide oxygen corresponding to fromabout 102 percent to about 110 percent of the calculated stoichiometricrequirement.

8. A process as claimed in claim2 in which all the lead values containedin the molten material withdrawn from the furnace are in the form oflead oxide.

9. A process as claimed in claim 2 in which the lead oxide content ofsaid material containing lead values is reduced to form substantiallysulphur-free lead.-

10. A process as claimed in claim 2 in which solid oxidic materialselected from the group consisting of reonto and penetrate the surfaceof a molten bath therein having lead values in the form of substantiallysulphide free lead oxide-containing slag derived from said procycledfume, flux-forming material and sulphate leach residues is charged intothe furnace in a stream that is in proximity to but not coincident withthe stream of lead sulphide ore or concentrate.

11. A. process as claimed in claim 1 in which said oxygen-rich gascontains at least percent oxygen.

12. A process as claimed in claim 1 in which said molten bath ismaintained at a temperature within the range of from about l,l00C. toabout 1,300C. by the heat of oxidation in the combustion zone.

13. A process as claimed in claim 2 in which said stream of leadsulphide ore or concentrate in particulate form and in admixture with anoxygen-rich gas is charged downwardly into the furnace at a distance ofat least about 36 inches above the slag cover,

14. A process as claimed in claim 1 in which said concentrate is a frothflotation product.

15. A process as claimed in claim 1 in which the temperature duringoxidation of the ore or concentrate in the combustion zone prior toimpingement upon the molten bath is above about 1,500C.

16. A process as claimed in claim 1 in which the temperature duringoxidation of the ore or concentrate in the combustion zone prior toimpingement upon the molten bath is about 1,700C.

17. A process as claimed in claim 1 in which the molten materialcontaining lead values withdrawn from the furnace contains less than 1percent sulphur and is withdrawn at an opening beyond the area ofimpingement.

18. A process as claimed in claim 1. in which said lead sulphide ore orconcentrate is charged in admixture with the oxygen-rich gas at avelocity sufficient to provide forceful impingement of said charge ontosaid slag cover for penetration of the surface thereof and accelerationof the oxidation reactions.

19. A process as claimed in claim 5 in which said lead oxide-containingslag is treated with a reducing agent for recovery of lead therefrom.

1. A PROCESS FOR SEPARATELY RECOVERING LEAD VALUES AND SULPHUR VALUESFROM A LEAD SULPHIDE ORE OR CONCENTRATE WITHOUT PRIOR SINTERING WHICHCOMPRISES: CHARGING LEAD SULPHIDE ORE OR CONCENTRATE IN PARTICULATE FROMAND IN ADMIXTURE WITH AN OXYGEN-FICH GAS CONTAINING AT LEAST ABOUT 60PERCENT OXYGEN DOWNWARDLY INTO AND THROUGH THE COMBUSTION ZONE OF AFURNACE TO IMPINGE ONTO AND PENETRATE THE SURFACE OF A MOLTEN BATHTHEREIN HAVING LEAD VALUES IN THE FORM OF SUBSTANTIALLY SULPHIDE FREELEAD OXIDE-CONTAINING SLAG DERIVED FROM SAID PROCESS, WHEREBY OXIDIATIONOF SAID LEAD SULPHIDE IS INITIATED AND SUSTAINED TO PRODUCE LEAD OXIDEAND SULPHUR DIOXIDE IN A FLAME IN SAID COMBUSTION ZONE ABOVE THE AREA OFIMPINGMENT ONTO SAID MOLTEN BATH AND, AFTER SUCH IMPINGEMENT, OXIDATIONOF RESIDUAL LEAD SULPHIDE IS CONTAINED BY REACTION WITHIN THE MOLTENSLAG; SUPPLYING IN THE FURNACE SUFFICIENT OXYGEN GAS IN ADMIXTURE WITHTHE ORE OR CONCENTRATE TO PROVIDE HEAT OF OXIDIATION TO MAINTAIN SAIDFLAME AT A TEMPERATURE OF AT LEAST 1,300*C. AND SAID MOLTEN BATH AT ATEMPERATURE ABOVE 1,100*C., AND SUFFICIENT TOTAL OXYGEN TO MAINTAIN ATLEAST 35
 2. A process as claimed in claim 1 in which said process isconducted continuously.
 3. A process as claimed in claim 1 in which fromabout 35 percent to about 55 percent lead as lead oxide is maintained inthe slag.
 4. A process as claimed in claim 1 in which lead valuescontained in the molten material withdrawn from the furnace include softlead containing less than 0.001 percent arsenic and less than 0.01percent antimony.
 5. A process as claimed in claim 2 said molten bathconsisting of lead and a substantially sulphide free leadoxide-containing slag cover, said oxygen-rich gas providing at leastabout 98 percent of the calculated stoichiometric requirement of oxygenfor treating said lead sulphide ore or concentrate to convert the leadsulphide completely to metallic lead and sulphur dioxide, and separatelywithdrawing from said furnace molten lead oxide-containing slag, moltenlead, and a mixture of gaseous material and entrained fume.
 6. A processas claimed in claim 2 in which said oxygen-rich gas is supplied in anamount sufficient to provide oxygen corresponding to from about 98percent to about 120 percent of the calculated stoichiometricrequirement.
 7. A process as claimed in claim 2 in which saidoxygen-rich gas is supplied in amount sufficient to provide oxygencorresponding to from about 102 percent to about 110 percent of thecalculated stoichiometric requirement.
 8. A process as claimed in claim2 in which all the lead values contained in the molten materialwithdrawn from the furnace are in the form of lead oxide.
 9. A processas claimed in claim 2 in which the lead oxide content of said materialcontaining lead values is reduced to form substantially sulphur-freelead.
 10. A process as claimed in claim 2 in which solid oxidic materialselected from the group consisting of recycled fume, flux-formingmaterial and sulphate leach residues is charged into the furnace in astream that is in proximity to but not coincident with the stream oflead sulphide ore or concentrate.
 11. A process as claimed in claim 1 inwhich said oxygen-rich gas contains at least 75 percent oxygen.
 12. Aprocess as claimed in claim 1 in which said molten bath is maintained ata temperature within the range of from about 1, 100*C. to abOut 1,300*C.by the heat of oxidation in the combustion zone.
 13. A process asclaimed in claim 2 in which said stream of lead sulphide ore orconcentrate in particulate form and in admixture with an oxygen-rich gasis charged downwardly into the furnace at a distance of at least about36 inches above the slag cover.
 14. A process as claimed in claim 1 inwhich said concentrate is a froth flotation product.
 15. A process asclaimed in claim 1 in which the temperature during oxidation of the oreor concentrate in the combustion zone prior to impingement upon themolten bath is above about 1,500*C.
 16. A process as claimed in claim 1in which the temperature during oxidation of the ore or concentrate inthe combustion zone prior to impingement upon the molten bath is about1,700*C.
 17. A process as claimed in claim 1 in which the moltenmaterial containing lead values withdrawn from the furnace contains lessthan 1 percent sulphur and is withdrawn at an opening beyond the area ofimpingement.
 18. A process as claimed in claim 1 in which said leadsulphide ore or concentrate is charged in admixture with the oxygen-richgas at a velocity sufficient to provide forceful impingement of saidcharge onto said slag cover for penetration of the surface thereof andacceleration of the oxidation reactions.
 19. A process as claimed inclaim 5 in which said lead oxide-containing slag is treated with areducing agent for recovery of lead therefrom.